The present invention relates to an apparatus for transferring a fluid, such as a pump. More particularly, the invention is concerned with an electromagnetic vibration pump which is adapted for transferring a fluid through a pipe by causing the pipe to vibrate in a manner like respiration.
Various types of pumps have been proposed for the purpose of transferring small amount of fluid. For instance, and electromagnetic pump has been known which has a diaphragm adapted to be vibrated by electromagnetic force so as to displace a small amount of fluid. On the other hand, Japanese Patent Laid-Open Nos. 9679/1981 and 68578/1984 propose a pump in which a cylindrically-shaped vibrator is directly vibrated. The fluid transferring effect in these known pumps relies upon a change in the volume of a chamber by an expansion or contraction of a part of a frame which forms the chamber, and does not necessitate any rotary or sliding parts such as an impeller and a piston. This type of pump, therefore, has a high reliability and is capable of transporting corrosive or highly viscous fluid which can hardly be handled by other types of pumps.
This type of pump, however, essentially requires provision of check valves at the inlet and outlet sides of the pump in order to prevent reversing of the fluid which may otherwise be caused by the periodical change in the volume. Since these check valves open and close in response to the displacement of the fluid, a pulsation is inevitably caused in the pressure of the fluid displaced by the pump. In some uses of the pump, the pulsation in the discharge pressure has to be avoided because it causes various troubles. It is well known that the pulsation of the displaced fluid can be suppressed by a pulsation prevention means such as a pressure accumulator. The use of such a pulsation prevention means, however, raises the cost of the pump system as a whole. According to another method, the vibration for causing the periodic change in the volume is conducted at a high frequency so as to shorten the period of the pulsation to such an extent that the pulsation is materially negligible. This method is advantageous in that it does not necessitate additional provision of any pulsation prevention means, but suffers from a problem in that the check valves which have movable masses cannot operate with good response to such a high frequency of pumping operation. Thus, there is a practical limit in the increase of the vibration frequency in the vibration pump of the kinds described, and it is impossible to shorten the period of pulsation unlimitedly.